1Field of the Invention
The present invention relates to a semiconductor heat sensitive switching device having PNPN junction.
2. Description of the Prior Art
It has been known to use semiconductor heat sensitive switching device for switching it by providing outer thermal energy instead of switching semi-conductor PNPN switching devices made of silicon etc., from the OFF state to the ON state by passing the gate current to the switching device in Japanese Patent Publication No. 33992/1972. (British Pat. No. 1,254,500 and West German Patent No. 1,943,193).
The typical semiconductor heat sensitive switching device is a heat sensitive thyristor having PNPN junction. A combination of transistor type heat sensitive device having PNP junction and one having NPN junction can be also used as the semiconductor heat sensitive switching device. The heat sensitive thyristor will be mainly illustrated.
In the specification, the minimum temperature required for switching the heat sensitive thyristor from the OFF state to the ON state under applying constant forward voltage (OF voltage) in the opening of the gate terminal is referred to as breakover temperature.
The minimum temperature required for extinction of forward blocking state (OFF state) in the main-voltage main-current characteristics in the opening of the gate terminal is referred to as inherent switch temperature. In order to decrease the range of the temperature for actuating the heat sensitive thyristor, there is a method of electrically decreasing it by utilizing the gate terminal. However, in many cases, it is desired to use it without such outer means by improving the heat sensitive thyristor itself to decrease the breakover temperature and the inherent switch temperature. The breakover temperature and the inherent switch temperature are respectively the temperature for providing unity of the sum of the common base current amplification factors .alpha..sub.1 and .alpha..sub.2 of the PNP transistor part and the NPN transistor part in the PNPN junction. In order to decrease the temperature, it is necessary to increase .alpha..sub.1 and .alpha..sub.2 at low temperature to provide unity of the sum thereof.
In the conventional thyristor for triggering by the gate current or luminescence, it is designed to have high breakover temperature for preventing thermal breakover. The breakover temperature is usually 100.degree. to 150.degree. C under applying the rated OFF state voltage and the inherent switch temperature is usually about 50.degree. C above the breakover temperature. When the thyristor having such temperature characteristics is used at lower temperature, it is necessary to apply the gate forward bias. In the method, the switch temperature is disadvantageously varied in high sensitive to the variation of the bias voltage.
Accordingly, the thyristor could not practically used as the heat sensitive switch.
In order to obtain a heat sensitive thyristor for switching it at a predetermined temperature without substantial effect caused by the variation of the applied voltage under lowering the switch temperature without the gate forward bias, it is necessary to have the characteristic of high temperature dependency of the reverse current of the collector junction (central junction) in the PNPN junction and the characteristic for rapidly providing unity of the sum of the common base current amplification factors .alpha..sub.1 and .alpha..sub.2 at low temperature by the increase of the collector current (rise of temperature).